Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, lithium-ion batteries that perform charge and discharge by transferring lithium ions between the positive and negative electrodes have been widely used as the driving power sources for the mobile information terminal devices.
The mobile information terminal devices tend to power consumption as the functions of the devices, such as moving picture playing functions and gaming functions. It is strongly desired that the lithium-ion batteries that are the drive power source for the devices have further higher capacities and higher performance in order to achieve longer battery life and improved output power.
Under these circumstances, the research and development efforts to provide lithium-ion batteries with higher capacities have been underway, which center around attempts to reduce the thickness of the battery can, the separator, or positive and negative electrode current collectors (e.g., aluminum foil or copper foil), as disclosed in Japanese Published Unexamined Patent Application No. 2002-141042, which are not involved in the power generating element, as well as attempts to increase the filling density of active materials (improvements in electrode filling density). These techniques, however, seem to be approaching their limits, and fundamental improvements such as finding alternative materials have become necessary to achieve a greater capacity in lithium-ion batteries. Nevertheless, regarding the attempts to increase the battery capacity through alternative positive and negative electrode active materials, there are few candidate materials for positive electrode active materials that are comparable or superior to the state-of-the-art lithium cobalt oxide in terms of capacity and performance, although alloy-based negative electrodes with Si, Sn, etc. appear to be promising as negative electrode active materials.
Under these circumstances, we have developed a battery with an increased capacity by raising the end-of-charge voltage of the battery, using lithium cobalt oxide as the positive electrode active material, from the currently common 4.2 V to a higher region to increase the utilization depth (charge depth). The reason why such an increase in the utilization depth can achieve a higher battery capacity may be briefly explained as follows. The theoretical capacity of lithium cobalt oxide is about 273 mAh/g, but the battery rated at 4.2 V (the battery with an end-of-charge voltage of 4.2 V) utilizes only up to about 160 mAh/g, which means that it is possible to increase the battery capacity up to about 200 mAh/g by raising the end-of-charge voltage to 4.4 V. Raising the end-of-charge voltage to 4.4 V in this way accomplishes about 10% increase in the overall battery capacity.
When lithium cobalt oxide is used at a high voltage as described above, the oxidation power of the charged positive electrode active material increases. Consequently, the decomposition of the electrolyte solution is accelerated, and moreover, the delithiated positive electrode active material itself loses the stability of the crystal structure. Accordingly, most important issues to be resolved have been the cycle life deterioration and the performance deterioration during storage due to the crystal disintegration. We have already found that addition of zirconia, aluminum, or magnesium to lithium cobalt oxide can achieve comparable performance to the 4.2 V battery even at a higher voltage under room temperature conditions. However, as recent mobile devices require higher power consumption, it is essential to ensure battery performance under high-temperature operating conditions so that the battery can withstand continuous operations in high temperature environments. For this reason, there is an imminent need to develop the technology that can ensure sufficient battery reliability even under high temperature conditions, not just under room temperature conditions.    [Patent Reference 1] Japanese Published Unexamined Patent Application No. 2002-141042